Most systems for the transmission and/or reception of information over wireless channels rely on some form of frequency control. For example, a wireless transmitter typically modulates information onto a radio-frequency carrier signal that is derived from a frequency reference generated locally by an oscillator. In order for a receiver to be able to receive the transmitted signal, it is desirable to keep the frequency of the carrier signal substantially constant, e.g. by controlling the frequency reference.
Oscillators are typically sensitive to temperature. Temperature transients affecting the output frequency of an oscillator may arise from such factors as changes in ambient temperature and local heating from nearby components. It may be desirable to apply some form of frequency control in order to prevent the frequency of the oscillator signal (and thus of the carrier signal) from drifting due to temperature transients.
Similarly, a wireless receiver typically receives a desired carrier signal having information modulated thereon by applying a frequency reference generated locally by an oscillator. In order for the receiver to continue to receive the transmitted signal, it may be desirable to keep a frequency of the frequency reference substantially constant. Like a transmitter, a receiver (which may be integrated with a transmitter, such as in a transceiver) may experience changes in oscillator frequency due to temperature transients. However, a receiver may also need to compensate for Doppler effects in a received signal.
Relative motion between a receiver and a transmission source (and/or apparent motion between the two, as might be caused by a moving reflector) causes a Doppler frequency error at the receiver that can be expressed in Hertz as vf/c cos φ, where v is the apparent relative velocity of the source with respect to the receiver, f is the carrier frequency in Hertz, c is the speed of light, and φ is the angle between the direction of travel of the receiver and the direction from the receiver to the transmission source. If the receiver is traveling directly toward the source, then φ=0, and if the receiver is traveling directly away from the source, then φ=π radians. For carrier frequencies in the gigahertz range and relative velocities of up to a few hundred miles per hour, the Doppler error may be as high as hundreds of Hertz, with a Doppler error of tens of Hertz being more typical for cases at lower relative velocities.
It is desirable to achieve frequency control in wireless communications systems that may experience temperature transients and Doppler error.